Multi-probe pressure test

ABSTRACT

A method and device for determining a formation gradient. The device comprises probes that are a set distance apart, disposing the device into a wellbore and measuring formation pressure with the probes provides formation pressures that can be correlated with the set distance. The measured pressures and correlated set distance is used to determine the formation gradient.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the field of exploration and production of hydrocarbons from wellbores. More specifically, the present invention relates to an apparatus and method for sampling connate fluid from a subterranean formation.

2. Description of Related Art

Sampling fluids entrained within a subterranean formation is one way of obtaining hydrocarbon producing potential of the formation. These fluids, also referred to as connate fluid, are typically analyzed in a laboratory. This sampling generally causes a minimum amount of damage to the tested formations. Additionally, a continuous record of the control and sequence of events during the test is made at the surface. From this record, valuable formation pressure and permeability data as well as data determinative of fluid compressibility, density and relative viscosity can be obtained for formation reservoir analysis.

Generally connate fluid sampling involves disposing a sonde 10 into a wellbore 5 via a wireline 8. Oppositely located on the outer portion of the sonde 10 usually are a sample port 14 and an urging means 12. When the sample port 14 is proximate to a formation of interest 6, the urging means 12 is extended against the inner surface of the wellbore 5 thereby engaging the sample port 14 into the formation 6. The engagement of the sample port 14 pierces the outer diameter of the wellbore 5 and enables fluid communication between the connate fluid in the formation 6 and the sample port 14. After inserting the sample port 14 into the formation 6, the connate fluid can be siphoned through the sample port 14 into the sonde 10 with a pumping means disposed therein.

Downhole multi-tester instruments have been developed with extendable sampling probes that engage the borehole wall and withdraw fluid samples from a formation of interest as well as measure pressure of the fluid within the formation. Traditionally these downhole instruments comprise an internal draw-down piston that is reciprocated hydraulically or electrically for drawing connate fluid from the formation to the instrument.

When exposed to an open hole, the fluid characteristics of formation fluid can change rapidly, thus it is important that the formation fluid be removed at least at the same flow rate as the formation fluid natural flow rate. However, it is important that the formation flow rate be regulated in order to prevent dropping the fluid pressure below its “bubble-point”. To be a representative sample the connate fluid must be in its original state without any portion of it allowed to bubble off. Should some of the lighter ends of the sample evaporate, these evaporated components cannot be easily recombined with the remaining portion of the sample. Thus even a sample with a small amount of evaporation results in an unrepresentative sample having altered fluid properties.

Connate fluid pressure in the formation can also be measured during the sampling process. One goal of measuring fluid pressure is to obtain a measurement of the pressure gradient within the formation 6. The pressure gradient represents the change in pressure per depth, this value is typically measured in psi/ft. The pressure increases generally in a linear fashion with respect to the depth. Pressure gradients may be obtained by first taking multiple pressure readings along the wellbore. The pressure readings are then correlated to the corresponding wellbore depths where each measurement was taken. Thus in order to obtain an accurate pressure gradient, the wellbore depths where each measurement was taken should be accurate. Often times however due to inherent uncertainties in wellbore measurements, the wellbore depth used in the pressure gradient calculation does not match the true vertical depth of where the measurements were obtained. These uncertainties are not limited to inaccurate true vertical depth readings, but extend also to incorrect differences in depth between adjacent measurement points, thereby resulting in an incorrect pressure gradient.

BRIEF SUMMARY OF THE INVENTION

The present invention includes a method of evaluating a formation gradient comprising, sampling the formation pressure at locations a set distance apart, and calculating the formation gradient based on the sampled formation pressure and the set distance. The step of sampling may be conducted with probes.

An optional method of sampling subterranean formation fluid is provided that comprises drawing a first amount of connate fluid from the formation, thereby creating a clean region in the formation and drawing a second amount of connate fluid from the clean region of the formation. The step of drawing amounts of connate fluid may be performed with probes. The probes may optionally be adjacently disposed.

Also disclosed herein is a fluid sampling system comprising probes disposed apart a set distance and an analyzer configured to calculate subterranean formation gradient based on a fluid pressure measurement of the probes and the set distance. The system may further comprise a body on which the probes are located, the probes may be configured to pierce the wall of a wellbore and sample fluid from a formation adjacent the wellbore.

Optionally, a connate sampling system is disclosed that comprises a sample probe and a cleaning probe. A pump in fluid communication with at least one of the probes may be included with the optional connate sampling system. The sampling system may further comprise a second cleaning probe as well as a body. The cleaning probe may be configured to selectively draw contaminated connate fluid and the sample probe may be configured to selectively draw clean connate fluid. The sampling systems may also comprise an information handling system.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 depicts a prior art downhole tool disposed in a wellbore.

FIG. 2 illustrates a side view of an embodiment of a downhole tool in a wellbore.

FIG. 3 illustrates a side view of an embodiment of a downhole tool in a wellbore.

FIGS. 4 a and 4 b are a detailed view of a portion of a downhole tool.

FIG. 5 is a partial cut-away side view of a downhole tool in a formation having compartments.

FIGS. 6 a-6 c represent pressure plots of formation compartments.

FIG. 7 is an overhead view of a downhole tool having probes at azimuthally varying locations.

DETAILED DESCRIPTION OF THE INVENTION

The apparatus and method described herein involves sampling subterranean connate fluid from within a formation that has been pierced by a wellbore. As is known, drilling subterranean wellbores produces a wall of mud cake is formed on the outer periphery of the wellbore. This wall generally extends along the length of the wellbore. Additionally, due to the drilling operations as well as drilling fluids introduced during drilling, large amounts of particulate matter may be contained within the connate fluid within the formation.

With reference now to FIG. 2 one embodiment of a portion of a downhole tool 16 is shown in a side view. The downhole tool 16 is shown disposed within a wellbore 18, where the wellbore 18 is in a cross sectional view. A layer of mudcake 22 is shown on the outer periphery of the wellbore 18. In this embodiment, the sampling tool 16 comprises sample probes (24, 28, 32, 36, and 40) disposed along the length of the tool 16. Each probe (24, 28, 32, 36, and 40) comprises a pad (25, 29, 33, 37, and 41) connected on one end to the body 17 with a corresponding probe tip (26, 30, 34, 38, and 42) disposed on the side of each pad opposite the body 17. The probe tips should include a conduit for allowing fluid flow therethrough. Through the pad, each probe tip is in fluid communication with a circuit provided in the body of the downhole tool 16. As shown, the pads (24, 28, 32, 36, and 40) are seated against the wellbore wall 19 with the probe tips (26, 30, 34, 38, and 42) extending through the mudcake 22 and into the formation 20. During use, the pad should be equipped to prevent leakage between the formation and the wellbore 18. To prevent leakage a seal may be disposed between the pad and the wellbore wall 19. Creation and use of a suitable pad is within the scope of capabilities of those skilled in the art.

As is known, the pads are disposed within or just adjacent to the tool housing when running the tool 16 within the wellbore 18. Recesses (not shown) may be formed onto the body 17 of the sampling tool 16 for receiving the probes while being moved in and out of the wellbore 18. Thus in order to sample the connate fluid the probes must be urged out and away from the body 17. This can be accomplished by any number of motivational means, such as by hydraulic, electrical, or spring loaded, to name but a few. It is within the capabilities of skilled artisans to develop an extension and retraction means for proper pad deployment and stowage.

In one mode of operation of the embodiment of FIG. 2, the downhole tool 16 is lowered into the wellbore 18 for evaluation of the formation 20. Evaluation may begin after the downhole tool 16 is at a depth in the wellbore 18 adjacent where the formation is to be probed. Once at the probing depth, the sample probes engage the wellbore wall 19 such that the probe tips penetrate the mudcake 22 and extend into the formation 20. In one mode of operation, connate fluid sampling comprises probing by individual sample probes at a time. For example, sampling may occur by inserting one sample probe 24 against the wellbore wall 19, sampling connate fluid, then retracting the sample probe 24. In a similar fashion, a second sample probe (one of 28, 32, 36, or 40) could be used to sample the connate fluid and so on until connate fluid samples are taken by one of, all, or some of the sample probes of the downhole tool 16.

As shown, urging means 21 are provided on the downhole tool 16 opposite the side of the sample probes. Thus engagement of the sample probes with the wellbore wall 19 can optionally occur by outwardly extending the urging means 21 against the wellbore wall 19 to provide an opposing force sufficient to pierce the wellbore wall 19. Moreover, the sample probe engagement shown in FIG. 2 can also include extending the sample probes from a recessed position within the downhole tool 16 into the wellbore wall 19. The urging means is not limited to the embodiment of FIG. 2, but can include any device or method capable of urging the downhole tool 16 against the wellbore wall 19.

By inserting the probe tips (26, 30, 34, 38, and 42) into the formation 20, connate fluid within the formation can be drawn from the formation into a tank or reservoir (not shown) in the downhole tool 16. One method of drawing connate fluid into the tool 16 comprises creating a differential pressure between the formation and the tool reservoir (or some other fluid receptacle) thereby forcing the fluid into the downhole tool 16. One or more pumps (not shown) can be provided within the downhole tool 16 configured to create such a differential pressure. The one or more pumps could selectively be in fluid communication with each probe tip such that only one, or more than one pump, drives the pressure differential. Optionally, a dedicated pump for each particular sample probe could be provided for the pumping action. Alternatively, the reservoir could be introduced into the wellbore 18 with an internal pressure sufficiently below the formation pressure to induce connate fluid flow therein.

Fluid communication between the formation 20 and the downhole tool 16 enables pressure measurement of the formation 20 by the downhole tool 16. The downhole tool 16 may be equipped with a pressure gauge or sensor dedicated for each sample probe. The gauge and sensor is used to measure the pressure. Optionally, the downhole tool 16 may have a single pressure gauge in communication with each sample probe. Other embodiments exist where a pressure gauge serves two or more sample probes in combination with one or more additional pressure gauges for the remaining probes.

The multiple sample probes arranged along the length of the downhole tool 16 provide multiple locations for discrete pressure measurement where each probe tip samples the connate fluid. The vertical distance between each pressure measurement location is equal to the vertical distance between the probe tips, thus the vertical distance between each pressure measurement location is known with accuracy and precision. The distance between the probe tips is known to a set distance before the tool 16 is disposed in the wellbore 18, moreover this set distance remains substantially the same during fluid sampling of the tool 16 in the wellbore 18. Thus while the probes are disposed in the wellbore 18, the distance between the probes is known with reliable accuracy.

As discussed above, a formation pressure gradient can be obtained by correlating the measured formation pressures with the corresponding measured relative depths. The correlation comprises taking the ratio of the changes in measured pressure along the set distances within the wellbore. A formation pressure gradient can therefore be accurately measured with the downhole tool 16 of FIG. 2 since the vertical distance between each pressure measurement location is set and precisely known. This distance, unlike wireline or tubing, does not change when inserted into a wellbore. An additional advantage of use of the present device is that once temperature stabilization within the downhole tool 16 has occurred, there is no need for temperature compensation between successive measures of the different sample probes.

It should be pointed out that the sample probes (24, 28, 32, 36, and 40) are not limited to being situated equidistance apart along the length of the downhole tool 16, but can be arranged such that the set distance can be any number of distance combinations on the tool. For example, the sample probe 24 and the sample probe 28 could be arranged such that their respective tips (26, 30) are at 1 vertical foot apart, whereas the sample probe 28 and the sample probe 32 could be arranged such that their respective tips (30, 34) are at 2 vertical feet apart. The remaining sample probes could be vertically spaced apart at the same distance, or a varying distance. FIG. 7 provides an overhead view of an embodiment of the downhole tool 16 a having sample probes (24 a, 28 a, 32 a, 34 a, 36 a, and 43) disposed at azimuthally distinct locations around the body of the tool 16 a. Additionally, the downhole tool 16 is not limited to the number of sample probes shown in FIG. 2, but can have as few as two and without an upper limit on the number of probes.

With regard to the term “vertically spaced” it could refer to the vertical distance along the tool itself, or the true vertical distance within the wellbore. In situations when the tool is disposed in a deviated section of a wellbore, a spacing of “X” (between adjacent sample probes) along the tool axis would translate into a vertical distance in the wellbore of some value less than “X”.

FIG. 3 illustrates an embodiment of a downhole tool 52 shown in a partial cross sectional view, where the downhole tool 52 is disposed within a wellbore 50. Optionally, the downhole tool 52 be disposed on wireline 54 as shown, but can also be deployed by other means, such as tubing, pipe string, slick line, and coiled tubing, to name but a few. The downhole tool 52 comprises a body 82 that may optionally house one or more of the following components, a tank 53, sampling devices (55, 67), pumps (57, 59), and supply headers (74, 83). Sample probes (60, 68, 76, 84, and 92) are included with the downhole tool 52 and are shown extending generally perpendicular to the axis of the body 82. Each probe (60, 68, 76, 84, and 92) comprises a pad (61, 69, 77, 85, and 93) connected on one end to the body 82 and disposed on the side of each pad opposite the body 82 is a corresponding probe tip (62, 70, 78, 86, and 94). In the embodiment of FIG. 3, lines (63, 71, 79, 83, and 95) are shown located within the body 82; each line is in fluid communication with a respective probe tip (62, 70, 78, 86, and 94). Each line includes a “tee” section where leads emanating from the tee section respectively terminate into a pair of supply headers (74, 83). Valves (64, 65, 72, 73, 80, 81, 88, 89, 96, and 97) are provided on the leads that provide for selective fluid communication between the lines (63, 71, 79, 83, and 95) and the supply headers (74, 83).

The supply header 83 is in fluid communication with a pump 57 for drawing connate fluid into the downhole tool 52. Optionally, each sample probe can be in communication with a dedicated pump for sampling connate fluid through the specific sample probe. The supply header 74 is in fluid communication with a pump 59 that is also configured to draw fluid from the formation 58 and into the downhole tool 52 for sampling purposes. A tank 53 is included shown in fluid communication with the supply header 83 via the pump 57.

Each sample probe (60, 68, 76, 84, and 92) on the downhole tool 52 of FIG. 3 is independently actuated from its recessed position into contact with the wall 90. Thus a single probe or any combination of two or more probes may be actuated at one time into wellbore wall 90 contact. In one mode of operation of the downhole tool 52 of FIG. 3, three of the sample probes (60, 68, and 84) are shown urged against the wellbore wall 90 wherein their respective probe tips (70, 78, and 86) extend through the wall 90 and mudcake 56 and into the formation 58. Once the probe tips (70, 78, and 86) have been inserted into the formation 58, one or both pumps (57, 59) can be activated for drawing connate fluid into the probe tips (70, 78, and 86) and into the lines (71, 79, and 87). Selective activation of the valves (72, 73, 80, 81, 96, and 97) determines into which supply header (74, 83) the connate fluid flows.

FIGS. 4 a and 4 b illustrate an optional mode of operation of the downhole tool 52, in this mode sample probes (68, 76, and 84) are shown engaged with the wellbore wall 90 with their respective tips (70, 78, and 86) penetrating through the mudcake 56 and into the formation. With reference now to FIG. 4 a, particular matter 51 is shown dispersed within the formation 58. Typically when connate fluid is drawn from a formation, some debris, such as the particulate matter shown, can be included with the connate fluid being sampled. This debris can contaminate the fluid being sampled and can also be responsible for clogs in the fluid sampling system.

FIG. 4 b illustrates a mode where sample probes (68, 84) adjacent the middle probe 76 are activated while no flow is induced through the middle probe 76 at that time. In doing so, the particulate matter 51 entrained within the connate fluid in the formation 58 is drawn towards the adjacent probes (68, 84). Pulling the particulate matter 51 from the formation in the formation fluid create a clean region within the formation 58, thus these probes (68, 84) are also referred to herein as “cleaning” probes. Subsequent activation of the probe 76 allows for connate fluid to flow from the clean region with no or little particulate matter to flow into and through the sample probe 76. Once drawn into the downhole tool 52, the valves can be selectively activated to divert the connate fluid from the cleaning probes into one of the supply headers and direct the fluid from the sample probe 76 into the other supply header. For example, the valve 73 and valve 90 could be in the open position whereas valve 72 and valve 88 could be in the closed position. This would send the connate fluid obtained through the sample probe 68 and the sample probe 84 to the supply header 74 thereby sending the connate fluid potentially carrying particulate matter to the pump 59 and sampling device 67. In this mode, the valve 80 should be in the open position and the valve 81 should be in the closed position, thereby directing “clean” sampled connate fluid into the supply header 83 for delivery to the sampling device 55 via the pump 57. This also allows the clean fluid to be directed to the tank 53 for storage and possible subsequent fluid analysis. It should be pointed out that the cleaning/sampling mode is not limited to the arrangement of FIGS. 4 a and 4 b, but any of the sample probes (60, 68, 76, 84 and 92) can be used as a sampling probe, and conversely any of these probes can also be used as a cleaning probe.

However the method of creating a clean region within the formation is not limited to the device of FIGS. 3, 4 a, and 4 b, but can be accomplished with additional probes, or even a single probe. If a single probe is used, the probe could draw down a first amount of fluid to create a “clean” region, direct that fluid to a first location, then be reinserted into the formation and draw out a second amount of fluid from the clean region, where the clean fluid is directed to a second location. Optionally two adjacent probes could be used in the cleaning and sampling process. Where one probe would function as a cleaning probe and the other as the sampling probe.

With reference now to FIG. 5, an embodiment of the downhole tool 16 is provided disposed within a wellbore 5 wherein the formation adjacent the wellbore 5 includes various compartments (44, 45, and 46). The adjacent compartment are each separated by a barrier 47, where the presence of the barrier 47 may provide a pressure seal that allows for differing pressure gradients between adjacent compartments. For example, FIGS. 6 a-6 c each contain a pressure gradient plot taken from within one of the compartments (44, 45 and 46). More specifically, FIG. 6 a graphically illustrates a pressure gradient plot representing compartment 44, FIG. 6 b graphically illustrates a pressure gradient plot representing compartment 45, and FIG. 6 c graphically illustrates a pressure gradient plot representing compartment 46. Each pressure gradient plot has a different slope indicating different fluid properties which in turn reveals the corresponding compartment. Thus implementation of the present device within a wellbore having compartments can indicate the presence of compartments as well as providing fluid data and fluid properties from within each compartment.

The recorded pressure measurement(s) may be stored within the downhole tool 16 for later analysis or can be transmitted to the surface, such as via wireline 5, for real-time analysis. The analysis includes determining the pressure gradient based on the measured pressures and the true vertical distance between adjacent measured pressures. An analyzer may be employed that is specifically configured to perform this analysis. The analyzer may be disposed with the downhole tool 16 or otherwise disposable within the wellbore 5. Optionally, the analyzer may be solely at the surface.

One specific example of an analyzer is an information handling system (IHS). An IHS may be employed for controlling the steps of measuring the pressure and upward and downward movement of the downhole tool 16 in the wellbore 5. Moreover, the IHS may also be used to store recorded measurements as well as processing the measurements into a readable format. The IHS may be disposed at the surface, in the wellbore, or partially above and below the surface. The IHS may include a processor, memory accessible by the processor, nonvolatile storage area accessible by the processor, and logics for performing each of the steps above described.

The present method described herein, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While a presently preferred embodiment of the invention has been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. For example, drawing or collecting connate fluid from a formation can be accomplished by any number of ways and is not limited to the use of the probes described herein. The methods and apparatus described herein could be accomplished by any manner of sampling fluid from within a subterranean formation. Additionally, the tools herein described can be combined with surface equipment for lowering/raising and controlling the tools within a wellbore. Examples of surface equipment includes surface trucks, information handling systems, as well as means for raising and lowering devices within the wellbore. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present invention disclosed herein and the scope of the appended claims. 

1. A method of evaluating a formation gradient comprising: sampling the formation pressure at locations a set distance apart; and calculating the formation gradient based on the sampled formation pressure and the set distance.
 2. The method of claim 1 wherein the step of sampling is conducted with probes.
 3. The method of claim 1, wherein the step of calculating the formation gradient comprises taking the ratio of the change in pressure at the locations a set distance apart.
 4. The method of claim 2, wherein the probes comprise two probes.
 5. The method of claim 2, wherein the probes comprise a multiplicity of probes.
 6. The method of claim 1, wherein the set distance is constant.
 7. The method of claim 1, wherein the set distance varies.
 8. The method of claim 1 further comprising sampling fluid from within the formation.
 9. The method of claim 7 further comprising analyzing the sampled fluid.
 10. The method of claim 1, wherein the set distance between adjacent locations from about 0.5 feet to about 5 feet.
 11. The method of claim 1, wherein the set distance between adjacent locations ranges from about 1 foot to about 2 feet.
 12. The method of claim 1, wherein the set distance between adjacent locations is about 1 foot.
 13. The method of claim 2, wherein the probes are on a downhole tool configured for connate fluid sampling.
 14. The method of claim 1 further comprising identifying a formation compartment.
 15. The method of claim 1 further comprising determining fluid properties within a formation compartment.
 16. A method of sampling subterranean formation fluid comprising: drawing a first amount of connate fluid from the formation, thereby creating a clean region in the formation; and drawing a second amount of connate fluid from the clean region of the formation.
 17. The method of claim 16, wherein the step of drawing amounts of connate fluid is performed with probes.
 18. The method of claim 17, wherein the probes are adjacently disposed.
 19. The method of claim 17 wherein the probes comprise cleaning probes disposed on opposite sides of a sampling probe.
 20. The method of claim 16, wherein the step of creating a clean region comprises drawing particulate matter entrained within the formation fluid.
 21. A fluid sampling system comprising: probes disposed apart a set distance; and an analyzer configured to calculate subterranean formation gradient based on a fluid pressure measurement of the probes and the set distance.
 22. The fluid sampling system of claim 21 further comprising a body on which the probes are located.
 23. The fluid sampling system of claim 21, wherein the probes are configured to pierce the wall of a wellbore and sample fluid from a formation adjacent the wellbore.
 24. The fluid sampling system of claim 21 further comprising a pump in fluid communication with the probes.
 25. The fluid sampling system of claim 21 wherein the set distance varies.
 26. The fluid sampling system of claim 21 wherein the set distance is substantially same.
 27. The fluid sampling system of claim 21 wherein the set distance ranges from about 0.5 feet to about 5 feet.
 28. The fluid sampling system of claim 21 wherein the set distance ranges from about 1 foot to about 2 feet.
 29. The fluid sampling system of claim 21, wherein the analyzer comprises an information handling system.
 30. The fluid sampling system of claim 21 further comprising surface equipment.
 31. A connate sampling system comprising: a sample probe; and a cleaning probe.
 32. The connate sampling system of claim 31 further comprising a pump in fluid communication with at least one of the probes.
 33. The connate sampling system of claim 31 further comprising a second cleaning probe.
 34. The connate sampling system of claim 31 further comprising a body.
 35. The connate sampling system of claim 31 wherein said cleaning probe is configured to selectively draw contaminated connate fluid.
 36. The connate sampling system of claim 31 wherein said sample probe is configured to selectively draw clean connate fluid.
 37. The connate sampling system of claim 31 further comprising an information handling system. 